This invention relates to a process and apparatus for the regeneration of a physical solvent loaded with sour gases, especially CO.sub.2 and H.sub.2 S.
To obtain a concentrated H.sub.2 S fraction in a selective, sour gas scrubbing process using a physical solvent, it is conventional to strip the solvent loaded with H.sub.2 S and CO.sub.2 in an enrichment column, under reduced pressure with an inert gas, normally N.sub.2, so that the main quantity of CO.sub.2 is desorbed and the more readily soluble H.sub.2 S remains at least partially in the solvent. At the same time, the H.sub.2 S is reabsorbed from the rising gaseous mixture in the upper section of the enrichment column by contact with a solvent free of H.sub.2 S. This technique results in an H.sub.2 S-free residual gas comprised primarily of CO.sub.2 and N.sub.2 as head product, and an H.sub.2 S-containing solvent as bottom product.
To obtain the H.sub.2 S fraction and/or for solvent regeneration, the solvent is freed of H.sub.2 S in a subsequent hot-regenerating column by a reboiler wherein the H.sub.2 S is withdrawn as a overhead stream.
It is possible, theoretically, to strip off increasing amounts of CO.sub.2 by correspondingly increasing the amount of stripping gas, thus attaining a greater H.sub.2 S concentration. However, this method cannot be applied to practical conditions, since with an increase in the amount of stripping gas, the amount of solvent at the head required for reabsorbing the H.sub.2 S is likewise increased automatically and consequently an increased amount of solvent must, in turn, be treated in the lower section of the column. Simultaneously, with an increase in the amount of stripping gas, other characteristics of the column change as well since, together with the CO.sub.2, also a greater quantity of H.sub.2 S is stripped off, whereby the H.sub.2 S-reabsorption section is subjected to a significant increase in the heat of solution.
It is known, as an alternative H.sub.2 S enrichment, to recycle part of the H.sub.2 S fraction from the head of the hot-regenerating column to the enrichment column and to reabsorb the H.sub.2 S from this fraction. However, in order to provide greater H.sub.2 S enrichment, a large portion of the head product from the hot-regenerating column must be recycled thereby resulting in considerable increases in refrigeration losses for the enrichment column (H.sub.2 S absorption), for the scrubbing agent vapor condensation, as well as in the thermal requirement for the hot-regenerating column (multiple driving out of CO.sub.2 and H.sub.2 S), and in the extent of saturation with scrubbing agent vapor in the recycled portion. Nevertheless, since, in many cases, the amount of nitrogen available as stripping gas is limited, for example, due to a previous design of the air fractionation unit for obtaining O.sub.2 for the partial oxidation of C-containing feedstocks, it has been necessary heretofore to additionally concentrate the H.sub.2 S fraction with respect to H.sub.2 S by means of the above-described recycling method.
It is also known to effect H.sub.2 S concentration even without the use of a stripping gas. In this case, the loaded solvent is expanded to a lower pressure to obtain a gaseous CO.sub.2 fraction. This fraction is then compressed to the pressure of a reabsorption column and the gaseous fraction is processed therein. In this process, the pressure in the expansion vessel is dependent on the required H.sub.2 S concentration in the H.sub.2 S fraction. In this connection, a high energy requirement must be met for the vacuum separation of the CO.sub.2 fraction, particularly at a low H.sub.2 S content in the raw gas or in case of a required high H.sub.2 S concentration in the H.sub.2 S fraction. Specific details of this process are found in U.S. Pat. No. 4,324,567.